qrjoint 2.1-0

• The C computational engine has been rewritten in Rcpp/RcppArmadillo, replacing the legacy .C() interface. qde() and qrjoint() now share a single underlying engine.
• Fixed a bug in qrjoint() (not qde()) where observations with zero weight (wt = 0) could produce NaN in the log-posterior, potentially freezing the MCMC chain. Zero-weight observations now correctly contribute nothing to the likelihood, as documented. This has no effect on models fit with the default (all nonzero) weights.
• Performance improvements, all exact (no change to the statistical model): Gaussian-process components are now cached and only recomputed for the MCMC blocks that actually change them; a redundant per-grid-point quantile-function evaluation in the base distribution is now computed once per log-posterior evaluation instead of once per grid point; and the per-observation quantile-bracket search now uses binary search instead of a linear scan. Combined, these give roughly a 1.7-1.9x speedup on typical problem sizes with identical statistical results.
• Added a more numerically robust Cholesky factorization for the adaptive proposal covariance, with jitter-based recovery on failure instead of silently propagating NaN.
• Updated the redmaple data source URL in the documentation.